config.ru: set RACK_ENV to deployment if RUBYWARDEN_ENV is production

If RACK_ENV is not set, which it may not be since RUBYWARDEN_ENV now
has to be used, it will default to development, which will cause
Sinatra to print stack traces and things to the public.

Protect users by setting RACK_ENV to "deployment" when
RUBYWARDEN_ENV is set to "production".

config.ru

···
 # OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 #
 
+if ENV["RUBYWARDEN_ENV"] == "production"
+  # prevent printing stack traces and other nonsense
+  ENV["RACK_ENV"] = "deployment"
+end
+
 require File.dirname(__FILE__) + "/lib/rubywarden.rb"
 require "#{APP_ROOT}/lib/app.rb"